Hydrostatic travelling drive

ABSTRACT

The invention relates to a hydrostatic travelling drive for a vehicle, having a primary-adjustable and secondary-adjustable hydrostatic driving gear, which comprises at least two hydraulic machines in the form of a hydraulic pump drivable by a drive motor of the vehicle and a hydraulic motor connected to the hydraulic pump by way of at least one operating line, both of which can be adjusted in their respective stroke volume in dependence upon a control signal changing with the load of the driving gear. To avoid disturbances of the secondary adjustment of the hydrostatic driving gear through momentarily or rapidly occurring fluctuations of the driving-gear load, as control signal for adjusting the hydraulic motor a travelling-speed signal generated by a sensor device and corresponding with the travelling speed of the vehicle is used which with increasing travelling speed effects a swinging-back of the hydraulic motor, i.e. an adjustment in the direction of its minimum stroke volume.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a hydrostatic travelling drive.

BACKGROUND OF THE INVENTION AND PRIOR ART

Travelling drives of this kind are known from practice and aredescribed, for example, in special print no. 1/79 (fluid February 1979),published by Hydromatik GmbH, for a wheel loader. The hydrostaticdriving gear of this known travelling drive, for the purpose of changingits transmission ratio, is primary-adjustable in a first travel range,i.e. by changing the stroke volume of the hydraulic pump, andsecondary-adjustable in a second travel range, i.e. by changing thestroke volume of the hydraulic motor, and consequently has a largeconversion range, which allows the application of large tractive forceson the one hand and the attainment of high travelling speeds on theother hand. Both the primary and the secondary adjustment take place independence upon a control pressure proportional to the speed of thedrive motor and changing with its load. In addition, a pressureregulation is provided, which adjusts the hydraulic motor in dependenceupon the operating pressure in the operating line, likewise changingwith the load, in such a way that it generates the respectively requiredtorque necessary to overcome the respective travel resistance.

This known travelling drive has proven itself in practice; although incertain travel situations it leads to superfluous and disturbingirregularities of the travelling speed. These are, for example, travelsituations with changes of the driving-gear load, for example due toload fluctuations of an operating or work drive connected to thetravelling drive or fluctuations of the travel resistance, which occurfor such a short period, for example upon the crossing of so-calledcorrugated-sheet tracks or potholes with the consequence of pressurepeaks in the operating line or slipping of drive wheels, that, due tothe speed-dependent control and pressure regulation used, they lead toan undesirably rapid response and adjustment of the hydrostatic drivinggear.

OBJECT OF THE INVENTION

The object of the invention is to develop a travelling drive in such away that disturbances of the secondary adjustment of its hydrostaticdriving gear through changing driving-gear loads of the type describedabove are avoided.

SUMMARY OF THE INVENTION

According to the present invention there is provided an hydrostatictravelling drive for a vehicle, having a primary-adjustable andsecondary-adjustable hydrostatic driving gear, which comprises at leasttwo hydraulic machines in the form of a hydraulic pump drivable by adrive motor of the vehicle and a hydraulic motor connected to thehydraulic pump by way of at least one operating line, both the hydraulicpump and hydraulic motor being adjustable in their respective strokevolume in dependence upon a control signal changing with the load of thedriving gear, wherein the control signal for adjusting the hydraulicmotor is a travelling-speed signal generated by a sensor device andcorresponding with the travelling speed of the vehicle, which signalwith increasing travelling speed effects a swinging-back of thehydraulic motor, i.e. an adjustment in the direction of its minimumstroke volume.

With the present invention the speed-dependent control and pressureregulation known in the prior art and the corresponding constructionaloutlay are omitted for the secondary adjustment, which is now effectedin dependence upon the travelling speed. In this way all disturbances ofthe secondary adjustment through momentary or rapid changes of thedriving-gear loads cease; only those load changes which generate acorresponding change of the travelling speed still affect the secondaryadjustment and lead to a change of the transmission ratio for thepurpose of adaptation to the respective load change.

Although not restricted thereto, the invention is particularly suitablefor mobile operating machines for off-road use, which, due to highercapacities and the use of mechanical switch gears, attain hightravelling speeds. In this respect, wheel loaders are to be mentioned inparticular, which with higher speeds are inclined to suffer pitchoscillations or "jumping" and which with their secondary adjustmentprovided in accordance with the invention travel with even speed whichis uninfluenced by the pitch oscillations.

The primary adjustment of the hydrostatic driving gear can likewise takeplace in dependence upon the travelling speed, but also upon the speedof the drive motor with the advantage of the control of the hydraulicpump and the hydraulic motor by means of two independent parameters.With use in rough terrain, whereby one usually drives in the firsttravel range, a rapid feedback of increased travel resistances can beattained through the so-called pressure regulation of the hydraulicpump; as long as the hydraulic motor remains set at maximum strokevolume, this pressure regulation is effective and adjusts the hydraulicpump in dependence upon a control signal derived from the operatingpressure in the operating line, whereby increasing operating pressureeffects an adjustment in the direction of minimum stroke volume.

The detection of the travelling speed can occur in different ways, forexample by way of a sensor according to the Doppler radar principle orby way of sensors such as measuring pumps, tacho-generators or pulsereceivers, which are advantageously arranged on non-driven wheels ormechanical driving gears.

Sensor devices which generate electrical signals are preferred. Thisenables the use of an electronic control unit which converts thereceived control signals into swivel-angle signals and, with these,controls the respective hydraulic machine for the purpose of adjustingits stroke volume. The conversion of the control signals takes place onthe basis of at least in each case one characteristic curve, stored inthe electronic control unit, of the swivel angle or of the appropriatestroke volume of each hydraulic machine in dependence upon the parameter(travelling speed, speed or operating pressure) picked up by therespective control signal.

According to a development of the invention the characteristic curves ofthe two hydraulic machines have such a common, specified intersectingpoint that they define a travel range in which the hydraulic pump andthe hydraulic motor are simultaneously controllable by the electroniccontrol unit. Favourable in this respect is a position of theintersecting point of the two characteristic curves such that the travelrange defined thereby is a second travel range of average or mediumtravelling speed, to which adjoin a first travel range of lowertravelling speed with sole control of the hydraulic pump and a thirdtravel range of higher travelling speed with sole control of thehydraulic motor. For the purpose of adaptation to different usageconditions the characteristic curves of the two hydraulic machines canhave different gradients and also a non-linear form.

The beginning of this second travel range can be displaced by settingthe intersecting point of the characteristic curves of the two hydraulicmachines in a setting device connected to the electronic control unit,by displacement of the characteristic curve of the hydraulic motor.

An advantageous development of the invention is characterized by a thirdsensor device for detecting the respectively set desired speed of thedrive motor and for emitting an appropriate control signal to theelectronic control unit, which unit contains a programme for detectinglabouring of the drive motor from the control signals of the second andthe third sensor device and with increasing labouring of the drive motordisplaces the characteristic curve of the hydraulic motor and thus thebeginning of the second travel range in the direction of a reduction ofthe same. In this way, upon use in, for example, rough terrain, thehydrostatic driving gear remains in the first travel range for theduration of the labouring of the drive motor, this being particularlyadvantageous when the pressure regulation is used and, if desired, theswivel-angle characteristic curve of the hydraulic pump is designed tobe flatter than that of the hydraulic motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail in the following withreference to the drawings.

FIG. 1 shows a block diagram of a wheel loader, the travelling drive ofwhich comprises a hydrostatic driving gear, the transmission ratio ofwhich can be adjusted according to the two embodiments of the invention,

FIG. 2 shows a diagram which represents the torque, delivered by thehydrostatic driving gear, across the entire conversion range, and

FIG. 3 shows a diagram which represents the characteristic curve of theswivel angle of each of the two hydraulic machines of the hydrostaticdriving gear according to FIG. 1 across the entire conversion range.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS ACCORDING TO THE INVENTION

The wheel loader represented in the drawing comprises a drive motor 1with adjustable speed, for example a diesel motor, a hydrostatictravelling drive, to which there is connected an operating drive, notshown, and a mechanical distributor gear 2 which is mechanicallyconnected by way of in each case a Cardan shaft 3 and in each case adifferential 4 to the drive shafts 5 of a front axle 6 and a rear axle 7with in each case two drive wheels 8.

The distributor gear 2 is provided with a first sensor device 11 in theform of a speed sensor for the purpose of detecting its output speedwhich is proportional to the travelling speed v of the wheel loader. Asecond and third sensor device 12 and 13, the former likewise in theform of a speed sensor and the latter in the form of a potentiometer,are provided for determining the speed n actually appearing and thedesired speed of the drive motor 1. An accelerator pedal 14 serves tospecify the desired speed, the respective position of which acceleratorpedal is determined by the potentiometer 13.

The hydrostatic travelling drive comprises a primary-adjustable andsecondary-adjustable hydrostatic driving gear with a hydraulic pump 16drivable by the drive motor 1 by way of a drive shaft 15, for example anaxial piston pump in swash-plate type of construction, and a hydraulicmotor 18 connected to the distributor gear 2 by way of an output shaft17, for example an axial piston motor in bent-axis type of construction,which is connected by way of two hydraulic operating lines 19, 20 to thehydraulic pump 16. The hydraulic pump 16 is a reversible adjustabledisplacement pump and, like the hydraulic motor 18 constructed asadjusting motor, has two directions of flow. Both hydraulic machines 16,18 are provided with a proportional-electrical control for the purposeof adjusting their stroke volume. In the case of the hydraulic motor 18this control comprises a proportional magnet 21 and a proportional valvecontrolled thereby, not shown, as well as an adjustment cylinder,likewise not shown, for which the adjusting pressure is taken from thehigh-pressure side of the hydraulic motor. The proportional-electricalcontrol for the hydraulic pump 16 corresponds in principle with thecontrol just described, but comprises two proportional magnets 22, 23,one for forward drive and one for reverse drive.

Both operating lines 19, 20 are connected by way of line sections to achange-over valve 24, from where a further line section leads to a p/A(pressure to electrical signal) converter 25.

The hydrostatic travelling drive comprises, furthermore, an electroniccontrol unit 26 which can also be constructed with microprocessor. Inputvariables for the electronic control unit 26 are the output speed of thedistributor gear 2, representative of the travelling speed v of thevehicle, the speed n of the drive motor 1, its desired speed, theoperating pressure in the hydrostatic driving gear and the respectivelydesired drive direction. To transmit the appropriate signals theelectronic control unit 26 is connected by way of signal lines 27 to 31to the first, second and third sensor devices 11, 12 and 13, to the p/Aconverter 25 and to a drive-direction switch 32.

As output variables the electrical control unit 26 supplies control(current) signals to the proportional magnets 21 to 23 of theproportional-electrical controls of the hydraulic machines 16, 18, inorder to adjust their swivel angle α and thus their stroke volume. Totransmit these control (current) signals, hereinafter calledswivel-angle signals, the electronic control unit 26 is connected by wayof signal lines 33 to 35 to the proportional magnet 21 of the hydraulicmotor 18 and to the proportional magnets 22, 23 of the hydraulic pump16.

The electronic control unit 26 is equipped with control circuit logicfor the primary and the secondary adjustment of the hydrostatic drivinggear and with a memory in which, according to a first embodiment of theinvention, there is stored the characteristic curve KL₁ of the swivelangel α of the hydraulic pump 16 in dependence upon the speed n of thedrive motor 1 and the appropriate characteristic curve KL₂ of thehydraulic motor 18 in dependence upon the travelling speed v. Accordingto a second embodiment of the invention the memory contains thecharacteristic curves of both hydraulic machines 16, 18 in dependenceupon the travelling speed v.

The characteristic curve KL₁ shows, taken together with FIG. 2, theprimary adjustment of the hydrostatic driving gear through adjustment ofthe hydraulic pump 16 which, starting from a swivel angle α_(o), isswung out from the value zero at idling speed n_(o) of the drive motor 1with increasing drive motor speed n (according to the 1st embodiment ofthe invention) or the travelling speed v (according to the 2ndembodiment of the invention) to its maximum swivel angle α_(max).Accordingly, the transmission ratio of the hydrostatic driving gearreduces, whereby in a first partial region up to the point P₁ of thetorque characteristic shown in FIG. 2 the torque delivered by thehydraulic pump 16 remains constant with constant operating pressureuntil reaching the maximum power of the drive motor 1 and in a secondpartial region drops hyperbolically to the point P₂ of the torquecharacteristic shown in FIG. 2 indicating the end of the primaryadjustment.

The characteristic curve KL₂ corresponds with the secondary adjustmentof the hydrostatic driving gear which takes place through adjustment ofthe hydraulic motor 18, which, starting from its maximum swivel angleα_(max) with increasing travelling speed v is swung back to its minimumswivel angle α_(min). With the appropriate reduction of the transmissionratio of the hydrostatic driving gear the torque delivered by thehydraulic motor 18 decreases to the point P₃ of the torquecharacteristic in FIG. 2.

The two characteristic curves Kl₁, KL₂ intersect in a commonintersecting point SP above the abscissa indicating the maximum swivelangle α_(max) of the hydraulic machines 16, 18 in FIG. 3 and define inthis way between their intersecting points with the abscissa a secondtravel range of average or middle travelling speed, in which bothhydraulic machines 16, 18 are simultaneously adjusted. The beginning ofthis second travel range, where the hydraulic pump 16 is swung out toabout 90% of its maximum stroke volume, is fixed by the travelling speedv₁. Its end, where the hydraulic pump 16 is swung back to the maximumswivel angle α_(max) and the hydraulic motor 18 to about 90% of itsmaximum stroke volume, is determined by the speed n₂ of the drive motor1 (according to the first embodiment of the invention) or by thetravelling speed v₂ (according to the second embodiment of theinvention).

Between the idling speed n_(o) and the beginning of the second travelrange at v₁ lies a first travel range of lower travelling speed, inwhich only the hydraulic pump 16 is adjusted. In the third travel rangeof higher travelling speed, adjoining the second travel range andreaching up to the maximum travelling speed v_(max) , only theadjustment of the hydraulic motor 18 takes place.

As can be seen from FIG. 3 (cf. broken lines extending parallel to KL₂),there is the possibility of arbitrarily displacing the characteristiccurve KL₂ of the hydraulic motor 18 within the first travel range. Forthis purpose a setting device 36 is provided, which is connected by wayof a partial section 29' of the signal line 29 to the third sensordevice 13 and by way of a second partial section 29" of the signal line29 to the electronic control unit 26. The setting device 36 allows,through setting of any desired travelling speed, for example v₁ ', whichis smaller than v₁, the fixing of the beginning of the second travelrange and thus the intersecting point, for example SP' of bothcharacteristic curves KL₁, KL₂ ', i.e. the reduction of the first travelrange and the enlarging of the second travel range accordingly. By wayof a line, not shown, the setting device 36 is connected to theelectronic control unit 26 and in reaction to the input of v₁ ' is setby this to that speed n₁ ' which corresponds with the specifiedtravelling speed v₁ ' when the drive motor 1 is not labouring, when,that is to say, the speed n actually appearing agrees with the desiredspeed specified by the accelerator pedal 14.

The operating behaviour of the hydrostatic travelling drive according tothe invention is explained in the following with reference to its firstembodiment.

When the wheel loader is at a standstill the drive motor 1 runs withidling speed n_(o). The hydraulic pump 16 is set at zero stroke volumeaccording to the swivel angle α_(o) and the hydraulic motor 18 tomaximum swivel angle α_(max). Through actuation of the accelerator pedal14 the speed n of the drive motor 1 is increased, detected by the speedsensor 12 and signalled by way of the signal line 28 to the electroniccontrol unit 26. Its circuit logic determines from the characteristiccurve KL₁ the respectively associated swivel angle α and controls withan appropriate control (current) signal that proportional magnet 22 or23 which is associated with the drive direction selected by thedrive-direction switch 32. The controlled proportional magnet 22 or 23swings the hydraulic pump 16 out to the swivel angle α determined by thecircuit logic. As soon as the hydraulic pump 16 delivers an adequateconveying flow, the hydraulic motor 18 begins to rotate and the wheelloader is set in motion. With further increasing speed n of the drivemotor 1 the hydraulic pump 16 is swung out further so that thetravelling speed v of the wheel loader increases. No control of theproportional magnet 21 of the hydraulic motor 18 takes place; the wheelloader travels in the first travel range.

Within the first travel range, with increased load of the hydrostaticdriving gear, for example due to higher travel resistances or the use ofthe operating drive, the so-called pressure regulation is put intoaction. As soon as the operating pressure in the respectivelyhigh-pressure-loaded operating line 19 or 20 exceeds a maximum valuespecified in the electronic control unit 26, the circuit logic controlsin accordance with a stored programme the proportional magnet 22 or 23with such a control (current) signal that the hydraulic pump 16 is swungback to the swivel angle necessary for keeping the specified maximumvalue of the operating pressure constant. In this way the drive power ofthe hydraulic pump is automatically adapted to the momentarily requiredhydraulic power. With reduction of the swivel angle α of the hydraulicpump 16 in the region of the torque curve located between P₁ and P₂according to FIG. 2 a torque increase results.

As soon as that the accelerator pedal 14 specifies the desired speedwhich corresponds with the speed n₁ ', to which the setting device 36 isset, the latter relays the appropriate desired-speed signal generated bythe potentiometer 13 to the electronic control unit 26, whichestablishes through comparison with the speed signal from the speedsensor 12, which determines the speed n of the drive motor 1 actuallyappearing, whether there is labouring of the drive motor 1. If this isnot the case, then the electronic control unit 26 controls theproportional magnet 21 with a swivel-angle signal which is determined byits circuit logic from the characteristic curve KL₂, starting from theoutput speed of the distributor gear 2, signalled by the speed sensor11, representative of the momentary travelling speed v over ground. Theproportional magnet 21 swings the hydraulic motor 18 back in thedirection of a smaller swivel angle. The second travel range has thusbegun; the pressure regulation of the hydraulic pump 16 is disconnected,both hydraulic machines 16, 18 are adjusted together, whereby it isensured that no premature travelling-speed state of equilibrium occurswhich prevents an increase of the travelling speed. As soon as thehydraulic pump 16 upon reaching the speed n₂ of the drive motor 1 is setat its maximum swivel angle α_(max), the second travel range ends andthe third travel range begins.

If, on the other hand, due to corresponding travel resistances there islabouring of the drive motor 1, the electronic control unit 26 delaysthe control of the proportional magnet 21 accordingly and in this waydisplaces the beginning of the second travel range in the direction ofhigher travelling speed, up to a maximum reaching v₁, at whichtravelling speed the hydraulic motor 18 according to the determinationin the electronic control unit 26 begins to swing back. By displacingthe beginning of the second travel range with laboring of the drivemotor 1 it is achieved that the vehicle travels, at maximum up to thetravelling speed v₁, in the first travel range of lower travellingspeed, in order to use the high output torque of the hydrostatic drivinggear and a rapid adaptation of this output torque to the respectivelyoccurring travel resistances through the pressure regulation, which isdiscontinued in the second and third travel range.

In the third travel range the hydraulic motor 18 is swung back in thesame manner as in the second travel range with increasing travellingspeed v and increasing output speed of the distributor gear 2 untilreaching its minimum swivel angle α_(min) and thus up to the maximumtravelling speed.

What is claimed is:
 1. Hydrostatic travelling drive for a vehicle havinga primary-adjustable and secondary-adjustable hydrostatic driving gear,which comprises at least two hydraulic machines in the form of ahydraulic pump drivable by a drive motor of the vehicle and a hydraulicmotor connected to the hydraulic pump by way of at least one operatingline, both the hydraulic pump and hydraulic motor being adjustable intheir respective stroke volume in dependence upon a control signalchanging with the load of the driving gear, wherein the control signalfor adjusting the hydraulic motor is a travelling-speed signal generatedby a sensor device and corresponding with the travelling speed of thevehicle, which signal with increasing travelling speed causes areduction in the stroke volume of the hydraulic motor, and wherein thecontrol signal for adjusting the hydraulic pump is a furthertravelling-speed signal generated by the sensor device, which signalwith increasing travelling speed causes an increase in the stroke volumeof the hydraulic pump;an electronic control unit for receiving therespective control signals and for controlling the hydraulic pump andthe hydraulic motor for the purpose of adjusting the same withswivel-angle signals, derived by the electronic control unit from thecontrol signals on the basis of at least one stored characteristic curveof the swivel angle of each hydraulic machine in association with anoperating parameter picked up by the respective control signals; thecharacteristic curves of both hydraulic machines having a common,specified intersecting point such that they define a travel range inwhich the hydraulic pump and the hydraulic motor are simultaneouslycontrollable by the electronic control unit; the travel range defined bythe intersecting point of the two characteristic curves being a secondtravel range of average or medium travelling speed, to which adjoin afirst travel range of lower travelling speed with sole control of thehydraulic pump and a third travel range of higher travelling speed withsole control of the hydraulic motor; and a setting device beingconnected to the electronic control unit for setting the intersectingpoint of the characteristic curves of the two hydraulic machines throughdisplacement of the characteristic curve of the hydraulic motor and thusof the beginning of the second travel range.
 2. Hydrostatic travellingdrive for a vehicle having a primary-adjustable and secondary-adjustablehydrostatic driving gear, which comprises at least two hydraulicmachines in the form of a hydraulic pump drivable by a drive motor ofthe vehicle and a hydraulic motor connected to the hydraulic pump by wayof at least one operating line, both the hydraulic pump and hydraulicmotor being adjustable in their respective stroke volume in dependenceupon a control signal changing with the load of the driving gear,wherein the control signal for adjusting the hydraulic motor is atravelling-speed signal generated by a sensor device and correspondingwith the travelling speed of the vehicle, which signal with increasingtravelling speed causes a reduction in the stroke volume of thehydraulic motor, and wherein the control signal for adjusting thehydraulic pump is a speed signal generated by a second sensor device andcorresponds with the speed of the drive motor of the vehicle, whichsignal at increasing speed of the drive motor causes an increase in thestroke volume of the hydraulic pump;an electronic control unit forreceiving the respective control signals and for controlling thehydraulic pump and the hydraulic motor for the purpose of adjusting thesame with swivel-angle signals, derived by the electronic control unitfrom the control signals on the basis of at least one storedcharacteristic curve of the swivel angle of each hydraulic machine inassociation with an operating parameter picked up by the respectivecontrol signals; and a third sensor device for determining therespectively set desired speed of the drive motor and for emitting anappropriate control signal to the electronic control unit, which unitcontains a program for detecting laboring of the drive motor from thecontrol signals of the second and the third sensor device and withincreasing laboring of the drive motor displaces the characteristiccurve of the hydraulic motor and thus the beginning of the second travelrange in the direction of a reduction of the same.
 3. Hydrostatictravelling drive for a vehicle having a primary-adjustable andsecondary-adjustable hydrostatic driving gear, which comprises at leasttwo hydraulic machines in the form of a hydraulic pump drivable by adrive motor of the vehicle and a hydraulic motor connected to thehydraulic pump by way of at least one operating line, both the hydraulicpump and hydraulic motor being adjustable in their respective strokevolume in dependence upon a control signal changing with the load of thedriving gear, wherein the control signal for adjusting the hydraulicmotor is a travelling-speed signal generated by a sensor device andcorresponding with the travelling speed of the vehicle, which signalwith increasing travelling speed causes a reduction in the stroke volumeof the hydraulic motor, and wherein the control signal for adjusting thehydraulic pump is a speed signal generated by a second sensor device andcorresponds with the speed of the drive motor of the vehicle, whichsignal at increasing speed of the drive motor causes an increase in thestroke volume of the hydraulic pump;an electronic control unit forreceiving the respective control signals and for controlling thehydraulic pump and the hydraulic motor for the purpose of adjusting thesame with swivel-angle signals, derived by the electronic control unitfrom the control signals on the basis of at least one storedcharacteristic curve of the swivel angle of each hydraulic machine inassociation with an operating parameter picked up by the respectivecontrol signals; a third sensor device for determining the respectivelyset desired speed of the drive motor and for emitting an appropriatecontrol signal to the electronic control unit, which unit contains aprogram for detecting laboring of the drive motor from the controlsignals of the second and the third sensor device and with increasinglaboring of the drive motor displaces the characteristic curve of thehydraulic motor and thus the beginning of the second travel range in thedirection of a reduction of the same; and wherein upon the setting ofthe hydraulic motor to maximum stroke volume the hydraulic pump isadjustable in dependence upon a control signal derived from theoperating pressure in the operating line, whereby increasing operatingpressure effects an adjustment in the direction of minimum strokevolume.
 4. Hydrostatic travelling drive for a vehicle having aprimary-adjustable and secondary-adjustable hydrostatic driving gear,which comprises at least two hydraulic machines in the form of ahydraulic pump drivable by a drive motor of the vehicle and a hydraulicmotor connected to the hydraulic pump by way of at least one operatingline, both the hydraulic pump and hydraulic motor being adjustable intheir respective stroke volume in dependence upon a control signalchanging with the load of the driving gear, wherein the control signalfor adjusting the hydraulic motor is a travelling-speed signal generatedby a sensor device and corresponding with the travelling speed, of thevehicle, which signal with increasing travelling speed causes areduction in the stroke volume of the hydraulic motor, and wherein thecontrol signal for adjusting the hydraulic pump is a speed signalgenerated by a second sensor device and corresponds with the speed ofthe drive motor of the vehicle, which signal at increasing speed of thedrive motor causes an increase in the stroke volume of the hydraulicpump;an electronic control unit for receiving the respective controlsignals and for controlling the hydraulic pump and the hydraulic motorfor the purpose of adjusting the same with swivel-angle signals, derivedby the electronic control unit from the control signals on the basis ofat least one stored characteristic curve of the swivel angle of eachhydraulic machine in association with an operating parameter picked upby the respective control signals; the characteristic curves of bothhydraulic machines having a common, specified intersecting point suchthat they define a travel range in which the hydraulic pump and thehydraulic motor are simultaneously controllable by the electroniccontrol unit; the travel range defined by the intersecting point of thetwo characteristic curves being a second travel range of average ormedium travelling speed, to which adjoin a first travel range of lowertravelling speed with sole control of the hydraulic pump and a thirdtravel range of higher travelling speed with sole control of thehydraulic motor; and a setting device being connected to the electroniccontrol unit for setting the intersecting point of the characteristiccurves of the two hydraulic machines through displacement of thecharacteristic curve of the hydraulic motor and thus of the beginning ofthe second travel range.
 5. Hydrostatic travelling drive according toclaim 2 or 3 or 4, wherein a third sensor device determines therespectively set desired speed of the drive motor and for emitting anappropriate control signal to the electronic control unit, which unitcontains a program for detecting laboring of the drive motor from thecontrol signals of the second and the third sensor device and withincreasing laboring of the drive motor displaces the characteristiccurve of the hydraulic motor and thus the beginning of the second travelrange in the direction of a reduction of the same.
 6. Hydrostatictravelling drive according to claim 1 or 2 or 3 or 4, wherein upon thesetting of the hydraulic motor to maximum stroke volume the hydraulicpump is adjustable in dependence upon a control signal derived from theoperating pressure in the operating line, whereby increasing operatingpressure effects an adjustment in the direction of minimum strokevolume.